i know this doesn't answer your question for any reason, however i think you've gotten a pretty good idea.
that said, i've gotten to the point where i don't even worry about it. if youre towing, just expect it to be bad. don't spend your drive worrying about your mileage or you'll kill any enjoyment of the ride.
it's going to cost extra money to travel this way, you're going to make more stops than you think, and you might find yourself wondering how far the next fuel stop is away from you.
that said, yep, you're going to burn extra fuel, just going in knowing and expecting to.